The genus Lasius is a conspicuous and popular genus of ants found in the Holarctic regions. We have completed the mitochondrial genome of Lasius spathepus as the first mitochondrial genome of Lasius. The mitochondrial genome is 18,951 bp long, which is the sixth longest ant mitochondrial genome known to science. It contains 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs, and a control region in a gene order shared with other species of subfamily Formicinae. The control region is 2,147 bp long, longest of all ants. Phylogenetic analysis shows L. spathepus groups with Nylanderia flavipes of the same tribe Lasiini.